Earth, a.k.a. Terra, Gaia or Sol III and referred to by many other names, is a medium-sized terrestrial planet and the third world orbiting the star Sol. It is orbited by a large, inhospitable moon known as Luna. Earth is perhaps best known for being the homeworld of the Human species, along with millions of other native lifeforms.

Contents

The Earth is believed to have been formed 4.5 billions of years ago, along with the rest of Sol's planetary system. During the course of its formation, it is believed that Earth collided with another planetary object, about the size of Mars, which resulted in the destruction of this object and the removal of a large chunk of the newborn Earth. This former portion of the planet stayed in orbit and eventually formed its moon, Luna.

After some half a billion years, Earth's surface cooled off, allowing for the formation of large oceans of liquid water. Some of the water had come to Earth via impacts of ice comets. The minerals dissolved in the oceans account for the sea salt. The first traces of life appeared soon afterwards. The planet's magnetic field is believed to have established itself about 3.5 billion years ago, helping to protect the ancient planet from solar radiation and other threats.

Exactly how life appeared on the planet is still a mystery. Some theories involve it having naturally evolved from complex chemical compounds in a primordial soup, this happening either near the ocean surface or near the hydrothermal sea vents in the deep sea, or still possibly in the atmosphere. Other theories state that life arrived on Earth from somewhere else, either naturally, as in the Panspermia Hypothesis; or with help from a sapient alien species, as in the Exogenesis Hypothesis.

Among the many sorts of unicellular creatures which evolved on Earth, some were phototrophs, using sunlight to perform photosynthesis, which involves the consumption of carbon dioxide and the ejection of free oxygen in the atmosphere. This oxygen was fatal for most of the lifeforms then, but some evolved to consume the new gas, using it for respiration. These oxygen-breathing microbes spread through the planet and gave origin to most of the extant earthling lifeforms.

During a poorly understood time period between 750 and 600 million years ago, the so-called Snowball Earth era, Earth was almost completely covered by water ice. It was only after this period, about 550 million years ago, that several multicellular animals evolved and spread in the world's oceans. Complex animals and plants later began to colonize the dry land at some point between 480 and 450 MYA.

During the last 400 million years, complex life underwent four major mass extinction events, with one of them, the Permian Extinction, resulting in the extinction of more than 90% of the planet's species. After that, a new group of reptiles emerged and some grew to enormous proportions. These creatures later became known as the dinosaurs and represent an iconic image of the prehistoric Earth. All of them, except for the bird family, were later eliminated by the next mass extinction, caused by a meteor impact 65 MYA. After the end of the dinosaurs' era the Earth was inherited mostly by mammals (although other vertebrate groups are also largely present, including birds, squamates, turtles, fish and amphibians). One mammalian species ended up developing ever increasing intelligence and technology and gave origin to the Human civilization. Although hotly debated, most scientists agree that many non-human animals have sapience to some extent. Among them, are all vertebrates and at least some invertebrates, such as cephalopods and arthropods.

Earth is the largest terrestrial planet of the Solar System and has the greatest density of any planet in the system as well. It is a geologically active planet, with an iron-nickel core surrounded by a mantle of liquid magma and covered by a cold rocky crust.

70% of the surface of Earth is covered by oceans of salt water. There are currently six main continents: North America, South America, Africa, Oceania, Antarctica and Eurasia. Some sources list Eurasia as two different continents: Europe and Asia; and some have the Americas taken together as one sole continent. Also, the lower portion of North America is sometimes called the Central America and treated as a continent on its own. However, all these views are mostly based on Human politics and sociology. Due to the constant movement of the continental plates, the continents' arrangement was different in each past period of Earth.

Earth also has two polar caps, one on the north, creating the frozen area known as the Arctic Ocean; and one on the south, blanketing the continent of Antarctica.

The atmosphere is composed of 78% nitrogen and 21% oxygen, which is produced by phototrophic lifeforms such as plants and phytoplankton. Several other elements are also present.

All life on Earth is carbon-based and has a very complex biochemistry involving several chemical elements, notably hydrogen, oxygen, silicon and nitrogen.

Main groups of organisms on the planet include: virus, bacteria, archaea (also known as archaeobacteria) and eukaryotes. The last group is believed to have been formed by means of symbiogenesis between bacteria and archaea. Within the eukaryotes several complex organism groups are found, including the phototrophic algae and land plants, the decomposing fungi and the predatory animals. Single-celled eukaryotes are also notable for their variety of forms and lifestyles, but are usually clumped together in the artificial group known as protozoa, nevertheless.

So far, Earth is the only place in the universe where life is known to exist, although this is most likely due to the technological difficulties in detecting lifeforms on other planets, or even in space vacuum, unless they showed themselves extremely close to Earth. According to our current knowledge it is perfectly conceivable that life may exist on several locations, either evolving there independently or spreading itself throughout the Universe, as dictates the hypothesis of Panspermia.

Several people across the Earth have related seeing alien spaceships (the so-called UFOs – Unidentified Flying Objects) or even interacting with aliens themselves. Some even believe that many extraterrestrials, belonging to such races as Greys or Reptoids, are secretly infiltrated in Human society. Many of these ufological reports have been proved as being either mistakes or hoaxes and the vast majority of scientists currently deny the possibility of alien visits or interactions with earthlings happening today. Some, however, like Erich von Däniken, have proposed that such encounters may have happened in the past, which would explain some of the world's supposed mysteries like the construction of enormous pyramids by allegedly primitive civilizations. These ideas too are largely denied by the scientific community due to the lack of evidences. The idea that some cryptic or mythological creatures, like the atmospheric beasts or the Chupacabras, may be of extraterrestrial origin has also been proposed by some.

Earth is definitely (and understandably) the planet most commonly featured in science-fiction media dealing with extraterrestrials, not always because of the planet itself, but rather because the presence of Humans – which are native to Earth, of course – provides the reader, viewer or player with a familiar element in the story.

Several sci-fi works feature Earth being visited, or invaded by sapient alien species of some sort. Others, set in the future, actually have Humans exploring other planets and regions and interacting with alien species of many sorts.

There are some examples of science-fiction works which do not mention Earth at all, most notably the Star Wars franchise, as well as the Walt Disney movie Treasure Planet, though both of which do feature Humans, without making mentions of the race's origins.

Sometimes Earth is portrayed as a thriving, highly developed, sometimes even near-utopian planet. In other universes, it is a nightmare world, often on the brink of or after an apocalypse.